Extrusion systems have been developed to form many types of products in the industrial, pharmaceutical and food industries. Most if not all of these extrusion systems have been designed to continuously form very high solids, very low moisture products by heating in the barrel of a single or double screw extruder and extruding through a die. In such cases, the formed product is held together upon extrusion by the self-adhesion of the almost-dry mass. An example of this is pasta, which is held together upon extrusion by the glutens in the flour. Another example is the simulated fruit pieces made by the method described in U.S. Pat. No. 5,084,296 (Lugay et al.). There, a mix containing gelling hydrocolloids and a minimal level of moisture is heated and then cooled in a single or double-screw extruder with different temperature zones. As a result of the low moisture content of the products produced in that fashion, the products tend to have relatively poor shape retention. Also, many food products lose their original flavor during processing and require the addition of flavors.
Another well-known process for producing shaped food and other products involves using chemical-setting gums such as alginates and/or cellulose ethers. For example, U.S. Pat. No. 4,436,759 (Trilling) discloses a process for extruding or molding a slurry of foodstuffs mixed with a chemical-setting binder. Although that patented process is an improvement on the prior methods that rely on chemical setting agents to form the food products, it does have the disadvantage of requiring very careful control of product hold times and also requires the use of specific chemical-setting gums which can interfere with the natural flavors of the products being shaped. In addition, the time required for the formation of the gel under such conditions is excessively long, thereby reducing efficiency or throughput.
It is also well-known that hydrocolloid gums such as gelatin, agars, carrageenans, gellan gum, pectins, alginates, etc., have an ability to bind high-moisture systems in a continuous gel matrix. Some of these gums, such as the alginates and low methoxyl pectins set into a gel upon exposure to specific chemicals, usually sources of calcium or similar bivalent ions. Others such as the gelatins, carrageenans, gellan gum and high methoxyl pectins set into a gel upon cooling following solubilization. Batch molding techniques using such hydrocolloid gums have been developed to form confectionery items. One example is the starch molding process in which impressions are made in a bed of compacted starch powder and then filled with a liquid containing solubilized thermal-setting gums, followed by cooling to set the gel. However, this method requires very expensive starch handling and fluid dispensing equipment and is not economical to use to form the small diameter gelled aqueous products with which we are concerned primarily here.
Until now there has been no way to form substantially continuously and economically small, gelled products with a high-moisture content, especially such products incorporating gums that gel upon cooling.